The present invention relates to a thin film actuated mirror array and more particularly to a thin film actuated mirror array having enhanced light efficiency by increasing the flatness of a reflecting member formed above actuating parts, and a method for manufacturing the same.
In general, light modulators are divided into two groups according to their optics. One type is a direct light modulator such as a cathode ray tube (CRT) and the other type is a transmissive light modulator such as a liquid crystal display (LCD). The CRT produces superior quality pictures on a screen, but the weight, the volume and the manufacturing cost of the CRT increase according to the magnification of the screen. The LCD has a simple optical structure, so the weight and the volume of the LCD are less than those of the CRT. However, the LCD has a poor light efficiency of under 1 to 2% due to light polarization. Also, there are some problems in the liquid crystal materials of the LCD such as sluggish response and overheating.
Thus, a digital mirror device (DMD) and actuated mirror arrays (AMA) have been developed in order to solve these problems. At the present time, the DMD has a light efficiency of about 5% and the AMA has a light efficiency of above 10%. The AMA enhances the contrast of a picture on a screen, so the picture on the screen is more apparent and brighter. The AMA is not affected by and does not affect the polarization of light and therefore, the AMA is more efficient than the LCD or the DMD.
The AMA is generally divided into a bulk type AMA and a thin film type AMA. The bulk type AMA is disclosed in U.S. Pat. No. 5,469,302 (issued to Dae-Young Lim). In the bulk type AMA, after a ceramic wafer which is composed of a multilayer ceramic inserted into metal electrodes therein is mounted on an active matrix having transistors, a mirror is mounted on the ceramic wafer by means of sawing the ceramic wafer. However, the bulk type AMA has disadvantages in that it demands a very accurate process and design, and the response of an active layer is slow. Therefore, the thin film AMA which is manufactured by using semiconductor technology has been developed.
The thin film AMA is disclosed in U.S. Pat. No. 5,815,305, dated Sep. 28, 1998, now U.S. Pat. No. 5,815,305 entitled "THIN FILM ACTUATED MIRROR ARRAY IN AN OPTICAL PROJECTION SYSTEM AND METHOD FOR MANUFACTURING THE SAME", is subject to an obligation to the assignee of this application.
FIG. 1 is a perspective view for showing the thin film AMA, FIG. 2 is a cross-sectional view taken along line A.sub.1 -A.sub.2 of FIG. 1, and FIG. 3 is a cross-sectional view taken along line B.sub.1 -B.sub.2 of FIG. 1.
Referring to FIGS. 1 and 2, the thin film AMA has a substrate 1, an actuator 90 formed on the substrate 1, and a reflecting member 80 installed on the actuator 90.
Referring to FIG. 2, the substrate 1 has an electrical wiring (not shown), a connecting terminal 5 formed on the electrical wiring, a passivation layer 10 formed on the substrate 1 and on the connecting terminal 5, and an etching stop layer 15 formed on the passivation layer 10. The electrical wiring and the connecting terminal 5 receive a first signal from outside and transmit the first signal. Preferably, the electrical wiring has a metal oxide semiconductor (MOS) transistor for switching operation. The passivation layer 10 protects the substrate 1 having the electrical wiring and the connecting terminal 5. The etching stop layer 15 prevents the passivation layer 10 and the substrate 1 from etching during subsequent etching steps.
The actuator 90 has a supporting layer 30 having a first portion attached to a portion of the etching stop layer 15 under which the connecting terminal 5 is formed and a second portion formed parallel to the etching stop layer 15, a bottom electrode 35 formed on the supporting layer 30, an active layer 40 formed on the bottom electrode 35, a top electrode 45 formed on the active layer 40, a common line 50 formed on the first portion of the supporting layer 30, and a post 75 formed on a portion of the top electrode 50. An air gap 25 is interposed between the etching stop layer 15 and the second portion of the supporting layer 30. The common line 50 is connected to the top electrode 50. The reflecting member 80 is supported by the post 75 so that the reflecting member 80 is formed parallel to the top electrode 50.
Referring to FIG. 3, the actuator 90 has a via contact 60 formed in a via hole 55 and a connecting member 70 formed from the via contact 60 to the bottom electrode 35. The via hole 55 is formed from a portion of the first portion of the supporting layer 30 to the connecting terminal 5. The bottom electrode 35 is connected to the via contact 60 via the connecting member 70. Therefore, the first signal, that is a picture signal, is applied to the bottom electrode 35 from outside through the electrical wiring, the connecting terminal 5, the via contact 60, and the connecting member 70. At the same time, when a second signal, that is a bias signal, is applied to the top electrode 45 from outside through the common line 50, an electric field is generated between the top electrode 45 and the bottom electrode 35. Thus, the active layer 40 formed between the top electrode 45 and the bottom electrode 35 is deformed by the electric field.
Preferably, the supporting layer 30 has a T-shape and the bottom electrode 35 has a rectangular shape. The bottom electrode 35 is formed on a central portion of the supporting layer 30. The active layer 40 has a rectangular shape which is smaller than the bottom electrode 35 and the top electrode 45 has a rectangular shape which is smaller than the active layer 40.
A method for manufacturing the thin film AMA will be described as follows.
FIGS. 4A and 4D illustrate the manufacturing steps of the thin film AMA in FIG. 2.
Referring to FIGS. 8A, at first, the substrate 1 having the electrical wiring (not shown) and the connecting terminal 5 is provided. Preferably, the substrate 1 is composed of a semiconductor such as silicon (Si). The connecting terminal 5 is formed by using tungsten (W). The connecting terminal 5 is connected to the electrical wiring. The electrical wiring and the connecting terminal 5 receive the first signal and transmit the first signal to the bottom electrode 35.
The passivation layer 10 is formed on the substrate 1 having the electrical wiring and the connecting terminal 5. The passivation layer 10 is formed by using phosphor-silicate glass (PSG). The passivation layer 10 is formed by a chemical vapor deposition (CVD) method so that the passivation layer 10 has a thickness of from 0.1 to 1.0 .mu.m. The passivation layer 10 protects the substrate 1 including the electrical wiring and the connecting terminal 5 during subsequent manufacturing steps.
The etching stop layer 15 is formed on the passivation layer 10 by using nitride so that the etching stop layer 15 has a thickness of from 1000 to 2000 .ANG.. The etching stop layer 15 is formed by a low pressure chemical vapor deposition (LPCVD) method. The etching stop layer 15 protects the passivation layer 10 and the substrate 1 during subsequent etching steps.
A first sacrificial layer 20 is formed on the etching stop layer 15 by using PSG so that the first sacrificial layer 20 has a thickness of from 0.5 to 2.0 .mu.m. The first sacrificial layer 20 enables the actuator 90 to form easily. The first sacrificial layer 20 is removed by using a hydrogen fluoride (HF) vapor when the actuator 90 is completely formed. The first sacrificial layer 20 is formed by an atmospheric pressure CVD (APCVD) method. In this case, the degree of flatness of the first sacrificial layer 20 is poor because the first sacrificial layer 20 covers the top of the substrate 1 having the electrical wiring and the connecting terminal 5. Therefore, the surface of the first sacrificial layer 20 is planarized by using a spin on glass (SOG) or by a chemical mechanical polishing (CMP) method. Preferably, the surface of the first sacrificial layer 20 is planarized by the CMP method.
After a portion of the first sacrificial layer 20 having the connecting terminal 5 formed thereunder is patterned along the column direction in order to expose a portion of the etching stop layer 15, a first layer 29 is formed on the exposed portion of the etching stop layer 15 and on the first sacrificial layer 20. The first layer 29 is formed by using a rigid material, for example a nitride or a metal so that the first layer 29 has a thickness of from 0.1 to 1.0 .mu.m. When the first layer 29 is formed by an LPCVD method, the ratio of nitride gas is adjusted according to the reaction time so as to release the stress in the first layer 29.
Referring to FIG. 4B, after a first photo-resist layer 32 is formed on the first layer 29 by a spin coating method, the first photo-resist 32 is patterned so as to expose a portion of the first layer 29 along the horizontal direction. As a result, a rectangular portion of first layer 29 which is adjacent to the connecting terminal 5 is exposed. After a bottom electrode layer is formed on the exposed portion of the first layer 29 and on the first photo-resist layer 32 by a sputtering method, the bottom electrode layer is patterned to form the bottom electrode 35 on the exposed portion of the first layer 29 considering the position on which the common line 50 will be formed. So, the bottom electrode 35 has a rectangular shape. The bottom electrode 35 is formed by using an electrically conductive metal such as platinum (Pt), tantalum (Ta) or platinum-tantalum (Pt-Ta) so that the bottom electrode 35 has a thickness of from 0.1 to 1.0 .mu.m.
A second layer 39 is formed on the bottom electrode 35 and on the first photo-resist layer 32. The second layer 39 is formed by using a piezoelectric material such as PZT (Pb(Zr, Ti)O.sub.3) or PLZT ((Pb, La)(Zr, Ti)O.sub.3) so that the second layer 39 has a thickness of from 0.1 to 1.0 .mu.m, preferably, about 0.4 .mu.m. Also, the second layer 39 is formed by using an electrostrictive material such as PMN (Pb(Mg, Nb)O.sub.3). The second layer 39 is formed by a sol-gel method, a sputtering method or a CVD method. Subsequently, the second layer 39 is annealed by a rapid thermal annealing (RTA) method. The second layer 39 will be patterned so as to form the active layer 40.
A top electrode layer 44 is formed on the second layer 39. The top electrode layer 44 is formed by using an electrically conductive metal such as aluminum (Al), platinum or tantalum. The top electrode layer 44 is formed by a sputtering method or a CVD method so that the top electrode layer 44 has a thickness of from 0.1 to 1.0 .mu.m.
Referring to FIG. 4C, after a second photo-resist layer (not shown) is coated on the top electrode layer 44 by a spin coating method, the top electrode layer 44 is patterned so as to from the top electrode 45 having a rectangular shape by using the second photo-resist layer as an etching mask. Then, the second photo-resist layer is removed by striping. The second layer 39 is patterned by the same method as that of the top electrode layer 44. That is, after a third photo-resist layer (not shown) is coated on the top electrode 45 and on the second layer 39 by a spin coating method, the second layer 39 is patterned so as to form the active layer 40 by using the third photo-resist layer as an etching mask. The active layer 40 has a rectangular shape which is wider than that of the top electrode 45. In this case, the active layer 40 is smaller than the bottom electrode 35. Then, the third photo-resist layer is removed by striping.
The first layer 29 is patterned so as to form the supporting layer 30 by the above-described method. The supporting layer 30 has a T-shape which differs from that of the bottom electrode 35. The bottom electrode 35 is formed on the central portion of the supporting layer 30.
The common line 50 is formed on the first portion of the supporting layer 30 after the first photo-resist layer 32 is removed. Namely, after a fourth photo-resist layer (not shown) is coated on the supporting layer 30 by a spin coating method and then the fourth photo-resist is patterned to expose the first portion of the supporting layer 30, the common line 50 is formed on the exposed portion of the supporting layer 30 by using an electrically conductive metal such as platinum, tantalum, platinum-tantalum or aluminum. The common line 50 is formed by a sputtering method or a CVD method so that the common line 50 has a thickness of from 0.5 to 2.0 .mu.m. At that time, the common line 50 is separated from the bottom electrode 135 by a predetermined distance and is attached to the top electrode 45 and to the active layer 40.
A portion of the first portion of supporting layer 30 having the connecting terminal 5 thereunder and a portion which is adjacent to the portion of the first portion of the supporting layer 30 are exposed when the fourth photo-resist is patterned. The via hole 55 is formed from the portion of the first portion of the supporting layer 30 to the connecting terminal 5 through the etching stop layer 15 and the passivation layer 10 by an etching. The via contact 60 is formed in the via hole 55 from the connecting terminal 5 to the supporting layer 30. At the same time, the connecting member 70 is formed on the portion which is adjacent to the portion of the first portion of the supporting layer 30 from the bottom electrode 35 to the via contact 60. Thus, the via contact 60, the connecting member 70, and the bottom electrode 35 are connected one after another. The via contact 60 and the connecting member 70 are formed by using an electrically conductive metal such as platinum, tantalum or platinum-tantalum. The connecting member 70 has a thickness of from 0.5 to 1.0 .mu.m. Thus, the actuator 90 having the top electrode 45, the active layer 40, the bottom electrode 35 and the supporting layer 30, is completed after the fourth photo-resist is removed by etching.
Referring to FIG. 4D, after the first sacrificial layer 20 is removed by using a hydrogen fluoride vapor, a second sacrificial layer 85 is formed on the actuator 90 by using a polymer having a fluidity. The second sacrificial layer 85 is formed by a spin coating method so that the second sacrificial layer 85 covers the top electrode 45. Subsequently, the second sacrificial layer 85 is patterned to expose a portion of the top electrode 45. The post 75 is formed on the exposed portion of the top electrode 45 and the reflecting member 80 is formed on the post 75 and on the second sacrificial layer 85. The post 75 and the reflecting member 80 are simultaneously formed by using a reflective metal such as aluminum, platinum or silver. The post 75 and the reflecting member 80 are formed by a sputtering method or a CVD method. Preferably, the reflecting member 80 for reflecting a incident light from a light source (not shown) is a mirror and has a thickness of from 0.1 to 1.0 .mu.m. The reflecting member 80 has a rectangular plate shape to cover the top electrode 45. The actuator 90 which the reflecting member 80 is formed thereon is completed as shown in FIGS. 1 and 2 after the second sacrificial layer 85 is removed by etching.
In the thin film AMA, the second signal is applied to the top electrode 45 through the common line 150 from outside. At the same time, the first signal is applied to the bottom electrode 35 through the electrical wiring, the connecting terminal 5, the via contact 60 and the connecting member 70 from outside. Thereby, an electric field is generated between the top electrode 45 and the bottom electrode 35. The active layer 40 formed between the top electrode 45 and the bottom electrode 35 is deformed by the electric field. The active layer 40 is deformed in the direction perpendicular to the electric field. The active layer 40 actuates in the direction opponent to the supporting layer 30. That is, the actuator 90 having the active layer 40 actuates upward by a predetermined tilting angle.
The reflecting member 80 for reflecting the incident light from the light source is tilted with the actuator 90 because the reflecting member 80 is supported by the post 75 and is formed on the actuator 90. Hence, the reflecting member 80 reflects the light onto the screen, so the picture is projected onto the screen.
However, in the above-described thin film AMA, the flatness of the reflecting member is lowered due to the deformation stress generated in the reflecting member for forming the reflecting member, so the light efficiency of the light incidented from the light source is decreased. That is, when the metal layer is deposited on the second sacrificial layer and patterned to form the reflecting member, the reflecting member may be bent upward or downward because of the deformation stress such as a residual stress caused by a compressive stress or a tensile stress for forming and patterning the metal layer, so the flatness of the reflecting member is lowered. As a result, the quality of the picture projected onto the screen is deteriorated according as the light efficiency of the reflecting member is decreased.